Earlier quoted context omitted.
My daughter at 12 got sucked into Tumblr and developed an eating disorder. I'm really sorry to hear this, having gone through a similar thing with a now ex-significant other. She too became very active in the segments of tumblr you identified and others (showed me many of them, some seemed like teenage angst in blog form, others were far more...worrisome), developed an eating disorder along with severe anxiety issues…
I think a lot of people over-estimate the resilience of the human emotional state, especially their own. Time and time again we see regular functioning adults get sucked into objectively insane or self-harmful communities, be it the above phenomenon or 4chan crap like QAnon.
What is the situation that makes these seem like a solid choice?